设置导航按钮的文字大小及颜色

  UIBarButtonItem *rightItem = [[UIBarButtonItem alloc] initWithTitle:@"完成" style:UIBarButtonItemStylePlain target:self action:@selector(nextAction)];
    self.navigationItem.rightBarButtonItem = rightItem;
    [rightItem setTitleTextAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:14]} forState:UIControlStateNormal];